This invention relates generally to space travel and, more particularly, to the selection of spacecraft trajectories and launch modes for interplanetary travel. The velocity at which a spacecraft must be launched from the earth's surface to attain a desired orbit about the earth is well known from principles of orbital mechanics. Similarly, the velocity at which a spacecraft must be launched from earth to reach a desired planetary target along a desired trajectory is also well known. The increment of velocity that has to be imparted to a spacecraft to attain a desired trajectory is often referred to as the .DELTA.V ("delta V").
Traditionally, spacecraft are placed into a desired trajectory by a multi-stage launch vehicle, the final or upper stage of which increases the velocity of the spacecraft sufficiently to achieve the total AV needed to reach the distant target. The high cost of launching a spacecraft is a major impediment to planetary exploration. Given that the cost of accelerating the spacecraft to the required .DELTA.V depends on the mass of the spacecraft, there is a practical limit to the spacecraft mass that can be sent to distant planets. One way of increasing this limit is to select a trajectory that takes advantage of the gravity assist of another planetary body. This trajectory is referred to as a planetary "swing-by," but has the disadvantage that the total transfer time is significantly increased in comparison to a more direct trajectory.
Accordingly, there is a need for an alternative approach to the design of interplanetary spacecraft missions, to increase the available payload from a given launch vehicle without increasing the transfer time, or to decrease the transfer time for a given payload. The present invention satisfies this need, as will become apparent from the following description.